For example, a rotation angle detector for detecting frequency of rotation of a rotating shaft is known (for example, refer to patent document 1). In this rotation angle detector, a magnet which is formed in disk shape is supported with a rotating shaft, and is maintained to be rotatable in the prescribed direction with the rotating shaft as the center. Furthermore, in the rotation angle detector, two magnetic sensors are provided. The two magnetic sensors consist of hall elements which have same heat characteristic each other and are arranged so that the line running through the center of the disk and one magnetic sensor meets the line running through the center of the disk and another magnetic sensor at an angle of 90°. And each magnetic sensor is arranged beneath the circumference of the magnet.
Patent document 1: Japanese Patent Application Publication 2003-75108 (P2-4, FIG. 4, 5)